Organic electroluminescent devices are known, for example from PCT/WO/13148 and U.S. Pat. No. 4,539,507. Such devices, as shown in FIG. 1, generally comprise a substrate 1, a first electrode 2 disposed over the substrate 1 for injecting charge of a first polarity; a second electrode 4 disposed over the first electrode 2 for injecting charge of a second polarity opposite to said first polarity; an organic light-emissive layer 3 disposed between the first and the second electrodes; and an encapsulant (not shown) disposed over the second electrode 4. In one arrangement, the substrate 1 and the first electrode 2 are transparent to allow light emitted by the organic light-emissive layer 3 to pass therethrough. Such an arrangement is known as a bottom-emitting device. In another arrangement, the second electrode 4 and the encapsulant are transparent so as to allow light emitted from the organic light-emissive layer 3 to pass therethrough. Such an arrangement is known as a top-emitting device.
Variations of the above-described structures are known. The first electrode may be the anode and the second electrode may be the cathode. Alternatively, the first electrode may be the cathode and the second electrode may be the anode. Further layers may be provided between the electrodes and the organic light-emissive layer in order to aid charge injection and transport. It is particular preferred to use a hole injecting layer and a hole transporting layer between the anode and the light-emissive layer. The hole injecting layer may comprise a conductive polymer such as PEDOT:PSS. The hole transport layer may comprise a semiconductive polymer such as a copolymer of fluorene and triarylamine repeat units. The organic light-emissive layer may comprise a small molecule, a dendrimer or a polymer and may comprise phosphorescent moieties and/or fluorescent moieties. The light-emissive layer may comprise a blend of materials including light emissive moieties, electron transport moieties and hole transport moieties. These may be provided in a single molecule or on separate molecules.
An example of such a device has a layer structure: substrate/ITO (140 nm)/PEDOT:PSS (65 nm)/hole transport layer (10 nm)/emissive layer (65-70 nm)/cathode.
By providing an array of devices of the type described above, a display may be formed comprising a plurality of emitting pixels. The pixels may be of the same type to form a monochrome display or they may be different colours to form a multicolour display. For example, a full colour display may be formed by providing sub-pixels of red, green and blue electroluminescent material.
By “red electroluminescent material” is meant an organic material that by electroluminescence emits radiation having a wavelength in the range of 600-750 nm, preferably 600-700 nm, more preferably 610-650 nm and most preferably having an emission peak around 650-660 nm.
By “green electroluminescent material” is meant an organic material that by electroluminescence emits radiation having a wavelength in the range of 510-580 nm, preferably 510-570 nm.
By “blue electroluminescent material” is meant an organic material that by electroluminescence emits radiation having a wavelength in the range of 400-500 nm, more preferably 430-500 nm.
A problem with organic electroluminescent devices is that much of the light emitted by organic light-emissive material in the organic light-emissive layer does not escape from the device. The light may be lost within the device by scattering, internal reflection, waveguiding, absorption and the like. This results in a reduction in the efficiency of the device. Furthermore, these optical effects can lead to low image intensity, low image contrast, ghosting and the like resulting in poor image quality.
A further problem with organic electroluminescent devices is that of achieving intense, narrow band-width emission so as to improve the colour purity of emission.
One way of solving the aforementioned problems is to utilize microcavity effects within a device.
A microcavity is formed when the organic light-emissive layer is disposed between two reflecting mirrors, one of which is semitransparent. The photon density of states is modified such that only certain wavelengths, which correspond to allowed cavity modes, are emitted with emission intensity being enhanced in a direction perpendicular to the layers of the device. Thus emission near the wavelength corresponding to the resonance wavelength of the cavity is enhanced through the semitransparent mirror and emission at wavelengths away from the resonance is suppressed.
Semitransparent mirrors are formed in a device at interfaces between layers having different refractive indices. The larger the difference between refractive indices, the more reflective the interface will be. Thus, interfaces which are formed between layers having very different refractive indices will be more optically active.
It is an aim of the present invention to increase out-coupling of light from an electroluminescent device by optimising the layer thicknesses within the electroluminescent device. It is a further aim of the present invention to increase optical out-coupling without adversely affecting the electrical properties of the device such that the overall opto-electrical efficiency of the device is increased. It is yet a further aim of the present invention to increase optical out-coupling and opto-electrical efficiency of the device without significantly altering the emission colour of the device. It is yet another aim to increase the lifetime of electroluminescent devices. Finally, it is an aim to achieve intense, narrow band-width emission so as to improve the colour purity of emission.